Material reduction machines in the nature of wood chippers are used to reduce wood to chips of various sizes, depending on their intended use. Generally, such a machine will comprise a reducing assembly having a rotating disk or drum with a plurality of cutting blades mounted around the periphery of the rotating disk or drum. The machine will include an enclosure for the reducing assembly, which enclosure will typically have a feed inlet through which wood materials such as logs and branches to be reduced are introduced, and a discharge outlet through which the chips are discharged after reduction. The use of wood chippers avoids the environmental and other problems associated with burning trees and brush for disposal, or with disposing of them in a landfill. Furthermore, by reducing wood to chips of a useful size, a wood chipper may be employed to produce a valuable chip product. Wood chips can be used as mulch or fuel. They can also be used as raw material for creating a pelletized fuel product or as raw material in a chemical pulp process.
Disk-type wood chippers include blades mounted on a rotating disk that cut across the grain of the wood stem generally perpendicular to the direction of the grain. Drum-type wood chippers include cutting blades mounted around the circumferential wall of a cylindrical chipper drum that cut across the wood feed stock in a path that varies with respect to the orientation of the grain of the feed stock to the drum.
Generally, a drum-type wood chipper includes a feed mechanism, a chipper drum that is partially enclosed in a housing, and a discharge chute. The chipper drum has a circumferential wall and is mounted for rotation on a shaft by means of an internal combustion engine or other driver engine. A plurality of cutting blades, each having a leading edge, are spaced around the circumferential wall of the chipper drum. As the drum rotates, the leading edges of the blades cut an arc that is concentric with and of a larger diameter than the circumferential wall of the drum. The feed mechanism carries woody material to be chipped into contact with the cutting blades on the chipper drum.
Uniformly sized chips are preferred for certain uses. The chips produced by a drum-type wood chipper will be of a generally uniform size if the rotational speed of the chipper drum, the extension of each blade from the circumferential wall of the drum, and the feed speed of material to the chipper drum are controlled so that the wood moves a precise distance toward the chipper drum during the time it takes for the drum to rotate from the completion of the cutting of a chip by a blade to the beginning of the cutting of a chip by the next blade mounted on the circumferential wall of the drum. However, the presentation of wood to the chipper drum by the feed mechanism causes resistance against the rotation of the chipper drum, and this resistance will slow the rate of rotation of the drum. When the rotational rate of the chipper drum slows, the operator of the wood chipper may reduce the rate of advance of wood to the chipper drum by the feed mechanism in an attempt to maintain the desired chip size. This feed mechanism speed response to resistance-induced chipper drum rotational rate changes is often effective for small changes in the rotational rate of the chipper drum. However, if the resistance created by the presentation of wood to the chipper drum is large or is generated rapidly, the operator of a conventional wood chipper may not be able to reduce the feed mechanism speed quickly enough to avoid stalling of the engine that drives the chipper drum. Consequently, conventional wood chippers are typically operated so that the feed mechanism will stop when resistance to the rotation of the chipper drum slows the driver engine to a predetermined lower operating rate. Then when the engine, operating under reduced load, recovers so that the rotational rate of the chipper drum returns to a desirable operational level, the feed mechanism will again be operated to advance wood towards the chipper drum. This method of operation is sometimes referred to as “chipping mode”. Operating a wood chipper in chipping mode results in chips of a uniform size, but at a lower production rate than would be obtained if it were not necessary to stop the advance of the wood to the chipper drum from time to time.
Another conventional method of operation is to operate the feed mechanism so as to reduce the rate of advance of wood to the chipper drum faster than the rate of reduction of the rotational rate of the chipper drum as the chipper drum slows due to resistance. This method of operation, which is sometimes referred to as “grinding mode”, will insure that the engine driving the chipper drum will not stall; however, chips produced using this method will not be uniform in size, even though the overall production quantity will be increased over what may be obtained using a chipping mode of operation.
It would be desirable if a method and apparatus for operating a drum-type wood chipper could be provided that would allow for operation at a higher production rate than a machine operating in chipping mode, while maintaining acceptable chip size uniformity.